1. Field of the Invention
The present invention relates to a control device for brushless motor and particularly to a control device capable of controlling rotation of a brushless motor without using rotor position detecting elements such as Hall elements.
2. Description of the Prior Art
Briefly stated, a brushless motor is a motor in which the position of magnetic poles of a rotor are detected by means of a detector directly coupled to the shaft of a rotor and in response to the detected positions, semiconductor switching elements such as transistors, thyristors and the like are turned on and off so as to continuously generate torque in the rotor. For the rotor, field winding or a permanent magnet is used.
FIG. 1 shows a conventional control device for a brushless motor. A brushless motor includes a stator 4 and a rotor 7. The stator 4 includes three-phase stator coils U, V, and W and rotor position detecting elements, for example, Hall elements H.sub.1, H.sub.2 and H.sub.3. The rotor 7 includes a permanent magnet having the surface on which magnetic poles N and S are formed. A control device comprises a single-phase AC power source 1, a rectifier circuit 2, a control circuit 5, an inverter 3 and a rotor position detector 6. The rectifier circuit 2 has for example a well-known thyristor bridge circuit and rectifies the alternating current power from the AC power source 1 so as to supply direct current power. The inverter 3 operates responsive to a control signal applied thereto to supply direct current power to the stator coils U, V, and W in predetermined modes so that a rotational magnetic field is generated in the stator 4. The inverter 3 includes, as switching elements, six transistors Q.sub.1 to Q.sub.6 connected in a three-phase bridge fashion. The switching elements are not limited to the transistors, and thyristors or other switching elements may be used. The rotor position detector 6 detects the rotation positions of the rotor 7 based on the signals from the Hall elements H.sub.1 to H.sub.3. The control circuit 5 controls conduction of the transistors Q.sub.1 to Q.sub.6 in the inverter 3 in a predetermined order in response to the signals from the rotor position detector 6.
The operation of such a control device for brushless motor will be described in more detail with reference to FIG. 2. FIG. 2 shows a timing chart in a control device for brushless motor. FIG. 2A shows the mode number and the rotation angle of a rotor. FIG. 2B shows the ON-OFF states of the transistors in an inverter. FIG. 2C shows a relation between voltage applied to the stator coils and voltage induced in the stator coils. The control circuit 5 controls the transistors Q.sub.1 to Q.sub.6 in the inverter 3 as shown in FIG. 2B in response to the signals, obtained from the rotation position detector 6, indicating the rotation positions of the rotor 7. More specifically, in the mode 1, the transistors Q.sub.1 and Q.sub.5 are turned on and the other transistors are turned off. As a result, electric current as shown by the arrow I in FIG. 1 flows between the stator coils U and V. In the mode 2, the transistors Q.sub.1 and Q.sub.6 are turned on and the other transistors are turned off. As a result, electric current as shown by the arrow II in FIG. 1 flows between the stator coils U and W. In the mode 3, the transistors Q.sub.2 and Q.sub.6 are turned on and the other transistors are turned off, whereby electric current as shown by the arrow III in FIG. 1 flows between the stator coils V and W. Subsequently in the same manner, in the modes 4 to 6, the transistors Q.sub.1 to Q.sub.6 are successively controlled as shown in FIG. 2B. Thus, a cycle of the modes 1 to 6 as described above is repeated.
In the following, description will be made of voltage applied to the stator coils U, V and W in each of the modes as described above. Referring to FIG. 2C, rectangular waveforms represent voltages applied to the stator coils U, V and W. In case where voltage on the positive side (the upper side from the central line in the drawing) is applied to the stator coils, electric current flows from the terminals of the stator coils on the power source side toward the neutral point N, and in case where voltage on the negative side (the lower side from the central line in the drawing) is applied to the stator coils, electric current flows from the neutral point N toward the terminals of the stator coils on the power source side.
When electric current flows in the stator coils U, V and W as described above, a rotational magnetic field is formed in the stator 4. Accordingly, a certain point .circle.M , for example, in the rotor 7 rotates from the point .circle.P to the points .circle.Q , .circle.R , .circle.X , .circle.Y and .circle.Z in FIG. 1 corresponding to the respective modes. Subsequently, corresponding to the modes successively repeated, the rotor 7 rotates, so that the motor can continue operation.
However, in a conventional brushless motor, Hall elements H.sub.1 to H.sub.3 are provided in three points in the stator 4 so as to detect the rotation positions of the rotor 7 and based on the detected positions, the control circuit 5 controls conduction, namely supply of current, to the stator coils, U, V and W. In such case, since the positions of the Hall elements are fixed, the angle of a brush as in a direct-current motor cannot be changed and therefore, it is difficult to make operation according to various load conditions in which a motor is to be used. In addition, the characteristics of a motor largely depend on the fixing positions of the Hall elements and accordingly, the fixing precision of the Hall elements becomes of great importance, which requires a high precision in manufacturing and assembling of the parts of a motor or in structure thereof. Furthermore, there are disadvantages that since heat resisting temperature of a Hall element is approximately 100.degree. C. or less, deterioration of the characteristics of the motor might be caused at the time of overload and consequently a limitation might be imposed on the operating conditions of the motor or troubles would be easily caused in the motor.
On the other hand, each Hall element needs four lead wires in all, that is, two lead wires for power source and the other two wires for output. Accordingly, in order to control a three-phase motor, 12 lead wires in all are needed for Hall elements H.sub.1 to H.sub.3 and in addition, three lead wires are needed for stator coils U, V, and W, which causes inevitably complication in the terminals for connection of a motor and lead wires, complicated positioning of the Hall elements in a stator, intricate control of precision in fixing the Hall elements to the stator or intricate control of connection of the lead wires between the respective elements and the control circuit. Furthermore, the Hall elements per se should be protected from oil, humidity and other environmental conditions in a place where the motor is installed, which necessarily imposes limitation on the application range of the motor.
Since conventional rotor position detecting elements such as Hall elements are fixed in positions determined in advance according to the application of the motor, for example, according to the driving speed of a load, the magnitude of the load and the like, there is a disadvantage that operation of high efficiency can not be performed in case where the motor is applied to the load or the rotational speed etc. different from the initially determined conditions.
In some motors, a method of controlling the operation utilizing voltage induced in stator coils is adopted conventionally. However, in such a conventional control method using induced voltage, macroscopic control is made by simply comparing the voltage induced in proportion to the number of revolutions with the reference voltage and accordingly, control of the rotation of a motor can not be made finely. In addition, since control of the conduction to the stator coils corresponding to the rotation positions of a rotor is not made, the operation of the motor is far from the optimum operation. Accordingly, in a conventional motor of this type, design had to be made by taking account of an allowance for driving power, which made it difficult to design the motor of a small and thin size.
As one method for dissolving the above described disadvantages, the Japanese Patent Publication Gazette No. 25038/1983 published on May 25, 1983 discloses a rotor position detecting circuit for brushless motor, in which, based on three comparison signals with phases deviating from each other by 120.degree., obtained by comparing the neutral voltage of the armature winding with respect to the negative power source side with the non-neutral three terminal voltages of the armature winding, group of switching elements of a semiconductor commutator apparatus is controlled so as to rotate a rotor including a magnet. However, this circuit needs a Miller integrator for obtaining control signals. Accordingly, it is needed to select an appropriate value for an integral constant of the Miller integrator (this is pointed out in the above stated gazette), and if the number of revolutions of the motor is out of the permissible range of the integral constant, control becomes impossible. In addition, because of a lag in control in the Miller integrator, step-out is caused by a rapid change in the load. Furthermore, since an accurate control signal cannot be obtained in the state where the motor starts to operate, other suitable means is needed for the start of operation (this is also pointed out in the above stated gazette). As one of such means, a cage conductor is provided in the rotor besides the magnet.
Therefore, a control device for brushless motor of other type not having such disadvantages has been desired.